Skin rashA good test, though one that may take a couple days, is to rub the plant on a section of arm or leg and see if you get a rash or whatnot. Rash = don't eat.

CookWhen you're initially trying to figure out if you can eat it or not, cook it.

Tasting the most likely poisonous substanceNo rash, not rotten and it doesn't smell like poison? Cook it. Put a small bit in your mouth. Bitterness is a bad sign. Burning of the tongue and mouth is a bad sign. Feeling like you're going to vomit is a bad sign. If none of these things are present, eat a small amount and wait several hours. If you don't die or feel terrible and you're still desperate then, my friend, you have found something to eat. Increase portion sizes slowly.

If you feel a sharp pain, drink warm water and don't eat anything until the pain subsides. If you become very sick and are near a fire pit crush up some charcoal and eat it. This will make you vomit and help neutralize the poison.

The advice given to CCF cadets regarding eating wild plants is, as I recall:

Rub a little on the the legs. The feet have thicker skin but if they become inflamed then you can't walk anywhere. Wait twelve hours. If there is no reaction, proceed to the next stage.

Rub the potential food on the wrists. This puts any poison present closer to the bloodstream, providing a more sensitive test. If there is still no reaction, proceed.

Put a little of the food on the tongue. Wait another twelve hours, for any reaction. If there is still no reaction, eat up!

You'll notice that this whole process takes up to 36 hours before you can actually eat anything. It is designed for you to begin as soon as you find yourself in a foreign environment, rather than when the food supplies are exhausted and the troops are starving.

The system is in typical British Army style: damage limitation. It'd have to be a pretty strong poison to take a man down just from contact with thick skin, and then the sensitivity of the tests are built up from there. The only risk would be if the test subject built up his own resistance, while the other troops would dive straight in after the trial period. That said, I don't know of any cases in which this occured.

Look at the sap (if sap is present) if you cut/break a plant stalk and discover a thin, milky sap then you've also likely discovered a toxic plant (dandelions are okay, bitter, but okay). Unfortunately, no one's ever told me what healthy sap should look like, so you're on your own there, sorry.

Seeing what other animals are eating is a good way of determining which plants are edible, but not always the best. Reindeer have been often known to eat various psilocybe (magic mushooms) at times, and seem to enjoy it almost as much as humans do. Most larger animals can break down some compounds much better than humans can, and while these plants probably won't kill you, they could make you very sick and if you're in a situation where you need to turn to wild plants for food, that's bad juju.
Of course, you may decide to go after the animal you were watching too, at the very least you'd get some exercise from trying to chase it down.